1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor layer being made mainly of silicon. The semiconductor device includes electronic elements such as a bipolar transistor and a field effect transistor and optical elements such as a photodiode.
2. Description of the Related Arts
In the field of semicondutor devices using silicon, silicon-germanium alloys comprising group IV elements have been studied as a semiconductor material, the monocrystal of which can be grown on monocrystalline silicon. A silicon-germanium alloy has an advantage in that the energy bandgap of the alloy can be monotonously varied by varying the germanium content thereof (see D.V. Lang et al., Applied Physics Letters, vol. 47, page 1333 (1986)). Further, the energy bandgap of a silicon-germanium alloy is smaller than that of monocrystalline silicon, so that various devices utilizing the difference between these energy bandgaps have been studied. For example, photodiodes using a multi-layer film comprising a monocrystalline silicon layer and a silicon-germanium layer and modified field effect transistors have been made on an experimental basis.
However, the monocrystal of a silicon-germanium alloy exhibits enhanced lattice mismatch against monocrystalline silicon as the germanium content thereof increases, thus causing distortion therein. Therefore, when monocrystalline silicon-germanium alloy having a high germanium content is grown on monocrystalline silicon, misfit dislocation due to this distortion occurs therein, which is a fatal defect for the application of the alloy to semiconductor devices. Particularly, when a thin monocrystalline silicon-germanium alloy layer is grown on monocrystalline silicon, a critical thickness for growing the monocrystal of the alloy without causing misfit dislocation comes into existence in relation to the germanium content of the silicon-germanium alloy layer.
This critical layer thickness decreases as the germanium content increases. For example, when the germanium content is 40%, which means that the difference in energy bandgap between monocrystalline silicon and silicon-germanium alloy is about 0.3 eV, the critical layer thickness of the thin alloy film has been reported to be 200 to 300 .ANG. (see R. People et al., Applied Physics Letters, vol. 47, page 322, (1985)).
Owing to the disadvantage as described above, there arises a technical problem that when silicon-germanium alloy monocrystal is grown on monocrystalline silicon in a thickness exceeding its critical value, too many misfit dislocations are generated in the alloy to obtain a thick film excellent in crystallinity. Meanwhile, in order to increase the critical layer thickness of a silicon-germanium alloy layer, the germanium content of the layer must be reduced, which brings about another technical problem that the use of silicon-germanium alloy having a desired energy bandgap becomes impossible.